Dewaxing and hydrodewaxing processes are principally employed in the refining industry to treat petroleum fractions having initial boiling points over about 350.degree. F. and relatively high pour points to improve (i.e., reduce) their pour points. The improvement in pour point is generally effected by selective removal of high boiling or waxy paraffinic hydrocarbons. Since the pour points of lubricating oils and liquid hydrocarbon fuels, e.g., diesel fuels and other light gas oil fractions, are strictly controlled, the pour point specifications of such lubricants and fuels must be met if such are to be employed in their intended use.
The need to reduce the pour point of petroleum fractions has resulted in the development of numerous dewaxing and hydrodewaxing processes wherein the pour points of petroleum fractions are reduced by selective removal of high boiling or waxy paraffinic hydrocarbons. Processes relating to dewaxing and hydrodewaxing are well known in both the patent and scientific literature. See, for example, Gary and Handwerk, PETROLEUM REFINING TECHNOLOGY AND ECONOMICS. Second Edition (Marcel Dekker, New York, 1984), pages 241 to 245, in which lubricating oils are described as being dewaxed by selective hydrocracking processes which crack the wax molecules to light hydrocarbons. Two types of selective hydrocracking processes are described -- one using a single catalyst for pour point reduction only (See Hargrave et al., Oil & Gas Journal Vol. 77(2), pp. 103-105, 1979), the other using two catalysts to reduce the pour point and improve the oxygen stability of the product (See Smith et al., Oil & Gas Journal Vol. 78(2), pp. 75-84, 1980). Both processes are said to employ for the pour point reduction operation synthetic shape-selective zeolite catalysts which selectively crack the n-paraffins and slightly branched paraffins. Zeolites with pore openings about 6 Angstroms in diameter are said to provide rapid cracking rates for n-paraffins, with the rates decreasing rapidly as the amount of branching increases. Certain zeolites are known to concentrate hydrocarbon reactants within the zeolite crystal; see applicant Rabo's "Unifying Principles in Zeolite Chemistry and Catalysis," Catalysis Review - Science & Engineering, Vol. 23, p. 299 (1981).
Such dewaxing processes have employed crystalline aluminosilicates as catalysts. For example, see U.S. Pat. Nos. 3,140,249, 3,140,252, 3,140,251, 3,140,253, 3,956,102 and 4,440,991. These and other patents disclose the use of various crystalline aluminosilicates as catalysts for dewaxing processes. U.S. Pat. No. 4,428,825 describes catalytic dewaxing as a method wherein a waxy feedstock is contacted with a suitable catalyst under conditions such that the waxy paraffins therein are cracked to form other hydrocarbons having less effect upon the pour point. The catalysts used (comprising silicalite) are said to be highly selective for cracking waxy components and often possess hydrogenation activity such that, in the presence of hydrogen, the cracked components are saturated by hydrogenation essentially immediately. Such catalysts are termed "hydrodewaxing" catalysts, and methods of dewaxing in such manner are known as "hydrodewaxing". Such processes are described in related U.S. Pat. No. 4,428,862 as preferably selectively hydrocracking and removing waxy paraffins while not substantially hydrocracking other components or substantially altering the boiling characteristics of the feedstock being treated, particularly with respect to the heavy fractions thereof.
Thus, for reducing the pour points of hydrocarbon feedstocks containing waxy or excessively high boiling components, processes and catalysts are needed which are capable of selectively converting these waxy components (i.e., generally n-paraffins of relatively high molecular weight and other components having very high boiling points) to achieve the necessary reductions in pour point (usually substantial, to produce products suitable for lubricating oils, fuel oils and the like) while leaving the boiling range and molecular weight distribution of the products substantially unchanged from those of the feedstock. While increasing amounts of the desired products as well as the waxy components may be converted under the relatively severe conditions required to produce large reductions in pour point, the objective is generally to achieve substantial reductions in pour point (at least to achieve product specifications) without substantial conversion of the nonwaxy components of the feedstock, since the latter represents a loss in yield.
Although a large number of zeolite materials have been disclosed as employable as catalysts for dewaxing and hydrodewaxing processes, the use of other crystalline molecular sieves has not received attention. This lack of attention is attributable to the scarcity of molecular sieves other than crystalline aluminosilicates. One disclosure of a catalyst containing a crystalline silicate, as opposed to a crystalline aluminosilicate, is found in U.S. Pat. No. 4,441,991. See also U.S. Pat. No. 4,428,825 and related U.S. Pat. No. 4,428,862, which utilize catalysts comprising silicalites.
The instant invention provides for catalytic dewaxing and hydrodewaxing of hydrocarbon feedstocks by contacting such with catalysts comprising silicoaluminophosphate molecular sieves, as hereinafter described.